One example of radio communication technology is a technology called WiMAX (Worldwide Interoperability for Microwave Access). The WiMAX is a technology developed as a method for constituting wireless MANs (Metropolitan Area Networks). The WiMAX connects telecommunication carriers and users by radio, instead of telephone lines or optical fiber lines, for example, to allow the MANs, which are wide area networks that interconnect LANs (local area networks) in urban areas or specific areas, to execute wireless transmission. It is said that this allows one radio base station (BS) to cover an area of about 50-km radius at a transmission speed of about 70 Mb/s at the maximum.
In radio access networks, such as the WiMAX, the coverage of radio communication is increased by disposing a plurality of BSs such that radio areas (for example, cells, sectors, etc.) provided by the BSs do not overlap as much as possible and a gap between the radio areas is as small as possible.
However, in places to which radio waves from BSs hardly reach, such as underground shopping areas, analog repeaters (ARs) are sometimes disposed instead of the BSs to expand the radio area at low cost. In such a case, to avoid communication interference between the ARs and the BSs, the ARs perform radio communication using, for example, a center frequency different from a radio frequency used by the BSs.
Thus, when a mobile station (MS) moves between the BSs and the ARs, the MS switches the frequency for use in radio communication.
A known example of technology for the above radio communication system is a method in which a network control management unit determines whether a radio resource available to BSs is greater than or equal to a first traffic radio resource calculated from a desired QoS (quality of service) of a MS, determines whether available radio resources of relay stations that relaying signals from those BSs determined to be true are greater than or equal to a second traffic radio resource calculated from the desired QoS, and controls the communication so that the MS can be connected to one of the relay stations determined to be true.
One example of a method for switching the frequency while a MS continues communication is hand-over (HO). Examples of the HO include controlled HO and uncontrolled HO.
With the controlled HO, the MS receives, for example, a neighboring-BS-information broadcast (MOB_NBR-ADV) message from a serving BS (SBS) at regular intervals, for example, once about 30 seconds. The MOB_NBR-ADV message includes information about a neighboring BS (NBS) neighboring the SBS (for example, information used for synchronizing with the NBS, such as a center frequency of the NBS).
When the MS detects that the intensity of radio waves from the SBS has become lower than a predetermined threshold value, the MS measures (scans) the intensity of radio waves from the NBS to search for a target BS (HO destination). When the intensity of the radio waves from the SBS further decreases to become lower than a HO threshold value, the MS negotiates with the SBS and hands over the connection to the NBS. Specifically, for example, the MS synchronizes (for example, frame-synchronizes) with the NBS and executes a connecting procedure called a network re-entry (terminal registering procedure) to take over the last connection state with the SBS (various kinds of communication information, etc.).
On the other hand, with uncontrolled HO, the MS executes the network re-entry process in synchronization with the NBS when communication with the SBS is disconnected without performing the above-described negotiation with the SBS.
With the uncontrolled HO, the NBS is not notified that the MS performs HO; therefore, the uncontrolled HO is different from the controlled HO in that replication of various kinds of communication information for use in starting communication (for example, context information) is started when the MS connects to the NBS. As a result, the uncontrolled HO requires more time for HO than the controlled HO.
With the above-described radio communication system having an AR, since the BS has no means for transmitting information on the AR to the MS, the MS cannot find the AR. Furthermore, since there is no means for knowing the master-slave relationship between a BS that manages the AR (reference BS) and the AR, a connection-destination switching procedure sometimes cannot be performed normally even if the AR is found.
For example, when a MS in communication with a SBS (radio frequency: f1) that manages an AR (radio frequency: f2) moves to the control of the AR, the MS receives a MOB_NBR-ADV message from the SBS; however, the MOB_NBR-ADV message does not include information on the AR. This is because the conventional MOB_NBR-ADV message contains only information on a NBS, and the AR is not a BS.
Accordingly, the MS cannot perform scanning at the radio frequency (f2) of the AR, so that the MS cannot find the AR as a HO destination in advance.
In this case, if the intensity of radio waves from the SBS falls below a HO threshold value, the MS becomes unable to communicate with the SBS, and therefore, the MS scans (searches) known radio frequencies. At that time, if no NBS or AR can be detected, the MS cannot perform HO, and thus, communication is sometimes disconnected. On the other hand, if the radio frequency (f2) of the AR can be detected by chance, the MS performs, for example, the above-described uncontrolled HO or an initial entry process. The initial entry process is a connection starting procedure that is performed when the MS cannot restart the communication with the SBS within a period during which the SBS holds information on the MS (resource retain time). However, if the MS performs the initial entry process, the time required for the communication process may be several times as much as the time required for HO.
When the MS performs the uncontrolled HO process, the MS executes the network re-entry process along with the HO. However, the target AR substantially communicates with the SBS, with the radio frequency different from that of the SBS (f1≠f2). Therefore, with the network re-entry process, the MS will perform duplicate connection to the SBS (double entry). This may cause mismatch of communication on the BS side or the MS side, which needs the network re-entry process to start reconnection, thus increasing the time required for the communication process.
As described above, when the MS switches communication destination in the radio communication system including the AR, the time required for the communication process sometimes increases.